Binaural rendering for headphones using metadata processing

ABSTRACT

Embodiments are described for a method of rendering audio for playback through headphones comprising receiving digital audio content, receiving binaural rendering metadata generated by an authoring tool processing the received digital audio content, receiving playback metadata generated by a playback device, and combining the binaural rendering metadata and playback metadata to optimize playback of the digital audio content through the headphones.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/098,268 filed on Nov. 13, 2020, which is a continuation of U.S.patent application Ser. No. 16/673,849 (now U.S. Pat. No. 10,838,684,issued Nov. 17, 2020) filed on Nov. 4, 2019, which in turn is acontinuation of U.S. patent application Ser. No. 16/352,607, filed onMar. 13, 2019 (now U.S. Pat. No. 10,503,461, issued Dec. 10, 2019),which in turn is a continuation of U.S. patent application Ser. No.15/934,849, filed on Mar. 23, 2018 (now U.S. Pat. No. 10,255,027, issuedApr. 9, 2019), which in turn is a continuation of U.S. patentapplication Ser. No. 15/031,953, filed on Apr. 25, 2016 (now U.S. Pat.No. 9,933,989, issued Apr. 3, 2018), which is the U.S. national stage ofInternational Patent Application No. PCT/US2014/062705 filed on Oct. 28,2014, which in turn claims priority to U.S. Provisional PatentApplication No. 61/898,365, filed on Oct. 31, 2013, each of which ishereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

One or more implementations relate generally to audio signal processing,and more specifically to binaural rendering of channel and object-basedaudio for headphone playback.

BACKGROUND

Virtual rendering of spatial audio over a pair of speakers commonlyinvolves the creation of a stereo binaural signal that represents thedesired sound arriving at the listener's left and right ears and issynthesized to simulate a particular audio scene in three-dimensional(3D) space, containing possibly a multitude of sources at differentlocations. For playback through headphones rather than speakers,binaural processing or rendering can be defined as a set of signalprocessing operations aimed at reproducing the intended 3D location of asound source over headphones by emulating the natural spatial listeningcues of human subjects. Typical core components of a binaural rendererare head-related filtering to reproduce direction dependent cues as wellas distance cues processing, which may involve modeling the influence ofa real or virtual listening room or environment. One example of apresent binaural renderer processes each of the 5 or 7 channels of a 5.1or 7.1 surround in a channel-based audio presentation to 5/7 virtualsound sources in 2D space around the listener. Binaural rendering isalso commonly found in games or gaming audio hardware, in which case theprocessing can be applied to individual audio objects in the game basedon their individual 3D position.

Traditionally, binaural rendering is a form of blind post-processingapplied to multichannel or object-based audio content. Some of theprocessing involved in binaural rendering can have undesirable andnegative effects on the timbre of the content, such as smoothing oftransients or excessive reverberation added to dialog or some effectsand music elements. With the growing importance of headphone listeningand the additional flexibility brought by object-based content (such asthe Dolby® Atmos™ system), there is greater opportunity and need to havethe mixers create and encode specific binaural rendering metadata atcontent creation time, for instance instructing the renderer to processparts of the content with different algorithms or with differentsettings. Present systems do not feature this capability, nor do theyallow such metadata to be transported as part of an additional specificheadphone payload in the codecs.

Current systems are also not optimized at the playback end of thepipeline, insofar as content is not configured to be received on adevice with additional metadata that can be provided live to thebinaural renderer. While real-time head-tracking has been previouslyimplemented and shown to improve binaural rendering, this generallyprevents other features such as automated continuous head-size sensingand room sensing, and other customization features that improve thequality of the binaural rendering to be effectively and efficientlyimplemented in headphone-based playback systems.

What is needed, therefore, is a binaural renderer running on theplayback device that combines authoring metadata with real-time locallygenerated metadata to provide the best possible experience to the enduser when listening to channel and object-based audio throughheadphones. Furthermore, for channel-based content it is generallyrequired that the artistic intent be retained by incorporating audiosegmentation analysis.

The subject matter discussed in the background section should not beassumed to be prior art merely as a result of its mention in thebackground section. Similarly, a problem mentioned in the backgroundsection or associated with the subject matter of the background sectionshould not be assumed to have been previously recognized in the priorart. The subject matter in the background section merely representsdifferent approaches, which in and of themselves may also be inventions.

BRIEF SUMMARY OF EMBODIMENTS

Embodiments are described for systems and methods of virtual renderingobject-based audio content and improved equalization in headphone-basedplayback systems. Embodiments include a method for rendering audio forplayback through headphones comprising receiving digital audio content,receiving binaural rendering metadata generated by an authoring tool,processing the received digital audio content, receiving playbackmetadata generated by a playback device, and combining the binauralrendering metadata and playback metadata to optimize playback of thedigital audio content through the headphones. The digital audio contentmay comprise channel-based audio and object-based audio includingpositional information for reproducing an intended location of acorresponding sound source in three-dimensional space relative to alistener. The method further comprises separating the digital audiocontent into one or more components based on content type, and whereinthe content type is selected from the group consisting of: dialog,music, audio effects, transient signals, and ambient signals. Thebinaural rendering metadata controls a plurality of channel and objectcharacteristics including: position, size, gain adjustment, and contentdependent settings or processing presets; and the playback metadatacontrols a plurality of listener specific characteristics including headposition, head orientation, head size, listening room noise levels,listening room properties, and playback device or screen positionrelative to the listener. The method may further include receiving oneor more user input commands modifying the binaural rendering metadata,the user input commands controlling one or more characteristicsincluding: elevation emphasis where elevated objects and channels couldreceive a gain boost, preferred 1D (one-dimensional) sound radius or 3Dscaling factors for object or channel positioning, and processing modeenablement (e.g., to toggle between traditional stereo or fullprocessing of content). The playback metadata may be generated inresponse to sensor data provided by an enabled headset housing aplurality of sensors, the enabled headset comprising part of theplayback device. The method may further comprise separating the inputaudio into separate sub-signals, e.g. by content type or unmixing theinput audio (channel-based and object-based) into constituent directcontent and diffuse content, wherein the diffuse content comprisesreverberated or reflected sound elements, and performing binauralrendering on the separate sub-signals independently.

Embodiments are also directed to a method for rendering audio forplayback through headphones by receiving content dependent metadatadictating how content elements are rendered through the headphones,receiving sensor data from at least one of a playback device coupled tothe headphones and an enabled headset including the headphones, andmodifying the content dependent metadata with the sensor data tooptimize the rendered audio with respect to one or more playback anduser characteristics. The content dependent metadata may be generated byan authoring tool operated by a content creator, and wherein the contentdependent metadata dictates the rendering of an audio signal containingaudio channels and audio objects. The content dependent metadatacontrols a plurality of channel and object characteristics selected fromthe group consisting of: position, size, gain adjustment, elevationemphasis, stereo/full toggling, 3D scaling factors, content dependentsettings, and other spatial and timbre properties of the renderedsound-field. The method may further comprise formatting the sensor datainto a metadata format compatible with the content dependent metadata toproduce playback metadata. The playback metadata controls a plurality oflistener specific characteristics selected from the group consisting of:head position, head orientation, head size, listening room noise levels,listening room properties, and sound source device position. In anembodiment, the metadata format comprises a container including one ormore payload packets conforming to a defined syntax and encoding digitalaudio definitions for corresponding audio content elements.

The method further comprises encoding the combined playback metadata andthe content dependent metadata with source audio content into abitstream for processing in a rendering system; and decoding the encodedbitstream to extract one or more parameters derived from the contentdependent metadata and the playback metadata to generate a controlsignal modifying the source audio content for playback through theheadphones.

The method may further comprise performing one or more post-processingfunctions on the source audio content prior to playback throughheadphones; wherein the post-processing functions comprise at least oneof: downmixing from a plurality of surround sound channels to one of abinaural mix or a stereo mix, level management, equalization, timbrecorrection, and noise cancellation.

Embodiments are further directed to systems and articles of manufacturethat perform or embody processing commands that perform or implement theabove-described method acts.

INCORPORATION BY REFERENCE

Each publication, patent, and/or patent application mentioned in thisspecification is herein incorporated by reference in its entirety to thesame extent as if each individual publication and/or patent applicationwas specifically and individually indicated to be incorporated byreference.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings like reference numbers are used to refer tolike elements. Although the following figures depict various examples,the one or more implementations are not limited to the examples depictedin the figures.

FIG. 1 illustrates an overall system that incorporates embodiments of acontent creation, rendering and playback system, under some embodiments.

FIG. 2A is a block diagram of an authoring tool used in an object-basedheadphone rendering system, under an embodiment.

FIG. 2B is a block diagram of an authoring tool used in an object-basedheadphone rendering system, under an alternative embodiment

FIG. 3A is a block diagram of a rendering component used in anobject-based headphone rendering system, under an embodiment.

FIG. 3B is a block diagram of a rendering component used in anobject-based headphone rendering system, under an alternativeembodiment.

FIG. 4 is a block diagram that provides an overview of the dual-endedbinaural rendering system, under an embodiment.

FIG. 5 illustrates an authoring tool GUI that may be used withembodiments of a headphone rendering system, under an embodiment.

FIG. 6 illustrates an enabled headphone that comprises one or moresensors that sense playback conditions for encoding as metadata used ina headphone rendering system, under an embodiment.

FIG. 7 illustrates the connection between a headphone and deviceincluding a headphone sensor processor, under an embodiment.

FIG. 8 is a block diagram illustrating the different metadata componentsthat may be used in a headphone rendering system, under an embodiment.

FIG. 9 illustrates functional components of a binaural renderingcomponent for headphone processing, under an embodiment.

FIG. 10 illustrates a binaural rendering system for rendering audioobjects in a headphone rendering system, under an embodiment.

FIG. 11 illustrates a more detailed representation of the binauralrendering system of FIG. 10 , under an embodiment.

FIG. 12 is a system diagram showing the different tools used in an HRTFmodeling system used in a headphone rendering system, under anembodiment.

FIG. 13 illustrates a data structure that enables delivery of metadatafor a headphone rendering system, under an embodiment.

FIG. 14 illustrates an example case of three impulse responsemeasurements for each ear, in an embodiment of a headphone equalizationprocess.

FIG. 15A illustrates a circuit for calculating the free-field soundtransmission, under an embodiment.

FIG. 15B illustrates a circuit for calculating the headphone soundtransmission, under an embodiment.

DETAILED DESCRIPTION

Systems and methods are described for virtual rendering of object-basedcontent over headphones, and a metadata delivery and processing systemfor such virtual rendering, though applications are not so limited.Aspects of the one or more embodiments described herein may beimplemented in an audio or audio-visual system that processes sourceaudio information in a mixing, rendering and playback system thatincludes one or more computers or processing devices executing softwareinstructions. Any of the described embodiments may be used alone ortogether with one another in any combination. Although variousembodiments may have been motivated by various deficiencies with theprior art, which may be discussed or alluded to in one or more places inthe specification, the embodiments do not necessarily address any ofthese deficiencies. In other words, different embodiments may addressdifferent deficiencies that may be discussed in the specification. Someembodiments may only partially address some deficiencies or just onedeficiency that may be discussed in the specification, and someembodiments may not address any of these deficiencies.

Embodiments are directed to an audio content production and playbacksystem that optimizes the rendering and playback of object and/orchannel-based audio over headphones. FIG. 1 illustrates an overallsystem that incorporates embodiments of a content creation, renderingand playback system, under some embodiments. As shown in system 100, anauthoring tool 102 is used by a creator to generate audio content forplayback through one or more devices 104 for a user to listen to throughheadphones 116 or 118. The device 104 is generally a portable audio ormusic player or small computer or mobile telecommunication device thatruns applications that allow for the playback of audio content. Such adevice may be a mobile phone or audio (e.g., MP3) player 106, a tabletcomputer (e.g., Apple iPad or similar device) 108, music console 110, anotebook computer 111, or any similar audio playback device. The audiomay comprise music, dialog, effects, or any digital audio that may bedesired to be listened to over headphones, and such audio may bestreamed wirelessly from a content source, played back locally fromstorage media (e.g., disk, flash drive, etc.), or generated locally. Inthe following description, the term “headphone” usually refersspecifically to a close-coupled playback device worn by the userdirectly over his or her ears or in-ear listening devices; it may alsorefer generally to at least some of the processing performed to rendersignals intended for playback on headphones as an alternative to theterms “headphone processing” or “headphone rendering.”

In an embodiment, the audio processed by the system may comprisechannel-based audio, object-based audio or object and channel-basedaudio (e.g., hybrid or adaptive audio). The audio comprises or isassociated with metadata that dictates how the audio is rendered forplayback on specific endpoint devices and listening environments.Channel-based audio generally refers to an audio signal plus metadata inwhich the position is coded as a channel identifier, where the audio isformatted for playback through a pre-defined set of speaker zones withassociated nominal surround-sound locations, e.g., 5.1, 7.1, and so on;and object-based means one or more audio channels with a parametricsource description, such as apparent source position (e.g., 3Dcoordinates), apparent source width, etc. The term “adaptive audio” maybe used to mean channel-based and/or object-based audio signals plusmetadata that renders the audio signals based on the playbackenvironment using an audio stream plus metadata in which the position iscoded as a 3D position in space. In general, the listening environmentmay be any open, partially enclosed, or fully enclosed area, such as aroom, but embodiments described herein are generally directed toplayback through headphones or other close proximity endpoint devices.Audio objects can be considered as groups of sound elements that may beperceived to emanate from a particular physical location or locations inthe environment, and such objects can be static or dynamic. The audioobjects are controlled by metadata, which among other things, detailsthe position of the sound at a given point in time, and upon playbackthey are rendered according to the positional metadata. In a hybridaudio system, channel-based content (e.g., ‘beds’) may be processed inaddition to audio objects, where beds are effectively channel-basedsub-mixes or stems. These can be delivered for final playback(rendering) and can be created in different channel-based configurationssuch as 5.1, 7.1.

As shown in FIG. 1 , the headphone utilized by the user may be a legacyor passive headphone 118 that only includes non-powered transducers thatsimply recreate the audio signal, or it may be an enabled headphone 118that includes sensors and other components (powered or non-powered) thatprovide certain operational parameters back to the renderer for furtherprocessing and optimization of the audio content. Headphones 116 or 118may be embodied in any appropriate close-ear device, such as open orclosed headphones, over-ear or in-ear headphones, earbuds, earpads,noise-cancelling, isolation, or other type of headphone device. Suchheadphones may be wired or wireless with regard to its connection to thesound source or device 104.

In an embodiment, the audio content from authoring tool 102 includesstereo or channel based audio (e.g., 5.1 or 7.1 surround sound) inaddition to object-based audio. For the embodiment of FIG. 1 , arenderer 112 receives the audio content from the authoring tool andprovides certain functions that optimize the audio content for playbackthrough device 104 and headphones 116 or 118. In an embodiment, therenderer 112 includes a pre-processing stage 113, a binaural renderingstage 114, and a post-processing stage 115. The pre-processing stage 113generally performs certain segmentation operations on the input audio,such as segmenting the audio based on its content type, among otherfunctions; the binaural rendering stage 114 generally combines andprocesses the metadata associated with the channel and object componentsof the audio and generates a binaural stereo or multi-channel audiooutput with binaural stereo and additional low frequency outputs; andthe post-processing component 115 generally performs downmixing,equalization, gain/loudness/dynamic range control, and other functionsprior to transmission of the audio signal to the device 104. It shouldbe noted that while the renderer will likely generate two-channelsignals in most cases, it could be configured to provide more than twochannels of input to specific enabled headphones, for instance todeliver separate bass channels (similar to LFE 0.1 channel intraditional surround sound). The enabled headphone may have specificsets of drivers to reproduce bass components separately from the mid tohigher frequency sound.

It should be noted that the components of FIG. 1 generally represent themain functional blocks of the audio generation, rendering, and playbacksystems, and that certain functions may be incorporated as part of oneor more other components. For example, one or more portions of therenderer 112 may be incorporated in part or in whole in the device 104.In this case, the audio player or tablet (or other device) may include arenderer component integrated within the device. Similarly, the enabledheadphone 116 may include at least some functions associated with theplayback device and/or renderer. In such a case, a fully integratedheadphone may include an integrated playback device (e.g., built-incontent decoder, e.g. MP3 player) as well as an integrated renderingcomponent. Additionally, one or more components of the renderer 112,such as the pre-processing component 113 may be implemented at least inpart in the authoring tool, or as part of a separate pre-processingcomponent.

FIG. 2A is a block diagram of an authoring tool used in an object-basedheadphone rendering system, under an embodiment. As shown in FIG. 2A,input audio 202 from an audio source (e.g., live source, recording,etc.) is input to a digital audio workstation (DAW) 204 for processingby a sound engineer. The input audio 201 is typically in digital form,and if analog audio is used, an A/D (analog-to-digital) conversion step(not shown) is required. This audio typically comprises object andchannel-based content, such as may be used in an adaptive audio system(e.g., Dolby Atmos), and often includes several different types ofcontent. The input audio may be segmented through an (optional) audiosegmentation pre-process, 204 that separates (or segments) the audiobased on its content type so that different types of audio may berendered differently. For example, dialog may be rendered differentlythan transient signals or ambient signals. The DAW 204 may beimplemented as a workstation for editing and processing the segmented orunsegmented digital audio 202, and may include a mixing console, controlsurface, audio converter, data storage and other appropriate elements.In an embodiment, DAW is a processing platform that runs digital audiosoftware that provides comprehensive editing functionality as well as aninterface for one or more plug-in programs, such as a panner plug-in,among other functions, such as equalizers, synthesizers, effects, and soon. The panner plug-in shown in DAW 204 performs a panning functionconfigured to distribute each object signal to specific speaker pairs orlocations in 2D/3D space in a manner that conveys the desired positionof each respective object signal to the listener.

In authoring tool 102 a, the processed audio from DAW 204 is input to abinaural rendering component 206. This component includes an audioprocessing function that produces binaural audio output 210 as well asbinaural rendering metadata 208 and spatial media type metadata 212. Theaudio 210 and metadata components 208 and 212 form a coded audiobitstream with binaural metadata payload 214. In general, the audiocomponent 210 comprises channel and object-based audio that is passed tothe bitstream 214 with the metadata components 208 and 212; however, itshould be noted that the audio component 210 may be standardmulti-channel audio, binaurally rendered audio, or a combination ofthese two audio types. A binaural rendering component 206 also includesa binaural metadata input function that directly produces a headphoneoutput 216 for direct connection to the headphones. For the embodimentof FIG. 2A, the metadata for binaural rendering is generated at mixingtime within the authoring tool 102 a. In an alternative embodiment, themetadata may be generated at encoding time, as shown with reference toFIG. 2B. As shown in FIG. 2A, a mixer 203 uses an application or tool tocreate audio data and the binaural and spatial metadata. The mixer 203provides inputs to the DAW 204. Alternatively, it may also provideinputs directly to the binaural rendering process 206. In an embodiment,the mixer receives the headphone audio output 216 so that the mixer maymonitor the effect of the audio and metadata input. This effectivelyconstitutes a feedback loop in which the mixer receives the headphonerendered audio output through headphones to determine if any inputchanges are needed. The mixer 203 may be a person operating equipment,such as a mixing console or computer, or it may be an automated processthat is remotely controlled or pre-programmed.

FIG. 2B is a block diagram of an authoring tool used in an object-basedheadphone rendering system, under an alternative embodiment. In thisembodiment, the metadata for binaural rendering is generated at encodingtime, and the encoder runs a content classifier and metadata generatorto generate additional metadata from legacy channel-based content. Forthe authoring tool 102 b of FIG. 2B, legacy multichannel content 220,which does not include any audio objects, but channel-based audio onlyis input to an encoding tool and rendering headphone emulation component226. The object-based content 222 is separately input to this componentas well. The channel-based legacy content 220 may first be input to anoptional audio segmentation pre-processor 224 for separation ofdifferent content types for individual rendering. In authoring tool 102b, the binaural rendering component 226 includes a headphone emulationfunction that produces binaural audio output 230 as well as binauralrendering metadata 228 and spatial media type metadata 232. The audio230 and metadata components 228 and 232 form a coded audio bitstreamwith binaural metadata payload 236. As stated above, the audio component230 usually comprises channel and object-based audio that is passed tothe bitstream 236 with the metadata components 228 and 232; however, itshould be noted that the audio component 230 may be standardmulti-channel audio, binaurally rendered audio, or a combination ofthese two audio types. When legacy content is input, the output codedaudio bitstream could contain explicitly separated sub-components audiodata or metadata implicitly describing content type allowing thereceiving endpoint to perform segmentation and process eachsub-component appropriately. The binaural rendering component 226 alsoincludes a binaural metadata input function that directly produces aheadphone output 234 for direct connection to the headphones. As shownin FIG. 2B, an optional mixer (person or process) 223 may be included tomonitor the headphone output 234 to input and modify audio data andmetadata inputs that may be provided directly to the rendering process226.

With regard to content type and the operation of the content classifier,audio is generally classified into one of a number of defined contenttypes, such as dialog, music, ambience, special effects, and so on. Anobject may change content type throughout its duration, but at anyspecific point in time it is generally only one type of content. In anembodiment, the content type is expressed as a probability that theobject is a particular type of content at any point in time. Thus, forexample, a constant dialog object would be expressed as a one-hundredpercent probability dialog object, while an object that transforms fromdialog to music may be expressed as fifty percent dialog/fifty percentmusic. Processing objects that have different content types could beperformed by averaging their respective probabilities for each contenttype, selecting the content type probabilities for the most dominantobject within a group of objects, or a single object over time, or someother logical combination of content type measures. The content type mayalso be expressed as an n-dimensional vector (where n is the totalnumber of different content types, e.g., four, in the case ofdialog/music/ambience/effects). The content type metadata may beembodied as a combined content type metadata definition, where acombination of content types reflects the probability distributions thatare combined (e.g., a vector of probabilities of music, speech, and soon).

With regard to classification of audio, in an embodiment, the processoperates on a per time-frame basis to analyze the signal, identifyfeatures of the signal and compare the identified features to featuresof known classes in order to determine how well the features of theobject match the features of a particular class. Based on how well thefeatures match a particular class, the classifier can identify aprobability of an object belonging to a particular class. For example,if at time t=T the features of an object match very well with dialogfeatures, then the object would be classified as dialog with a highprobability. If, at time=T+N, the features of an object match very wellwith music features, the object would be classified as music with a highprobability. Finally, if at time T=T+2N the features of an object do notmatch particularly well with either dialog or music, the object might beclassified as 50% music and 50% dialog. Thus, in an embodiment, based onthe content type probabilities, audio content can be separated intodifferent sub-signals corresponding to the different content types. Thisis accomplished, for example, by sending some percentage of the originalsignal to each sub-signal (either on a wide-band basis or on a perfrequency sub-band basis), in a proportion driven by the computed mediatype probabilities.

With reference to FIG. 1 , the output from authoring tool 102 is inputto renderer 112 for rendering as audio output for playback throughheadphones or other endpoint devices. FIG. 3A is a block diagram of arendering component 112 a used in an object-based headphone renderingsystem, under an embodiment. FIG. 3A illustrates the pre-processing 113,binaural rendering 114, and post-processing 115 sub-components ofrenderer 112 in greater detail. From the authoring tool 102, themetadata and audio are input into processing or pre-processingcomponents in the form of a coded audio bitstream 301. The metadata 302is input to a metadata processing component 306, and the audio 304 isinput to an optional audio segmentation pro-processor 308. As shown withreference to FIGS. 2A and 2B, audio segmentation may be performed by theauthoring tool through pre-processors 202 or 224. If such audiosegmentation is not performed by the authoring tool, the renderer mayperform this task through pre-processor 308. The processed metadata andsegmented audio is then input to a binaural rendering component 310.This component performs certain headphone specific rendering functions,such as 3D positioning, distance control, head size processing, and soon. The binaural rendered audio is then input to audio post-processor314, which applies certain audio operations, such as level management,equalization, noise compensation or cancellation, and so on. Thepost-processed audio is then output 312 for playback through headphones116 or 118. For an embodiment in which the headphones or playback device104 are fitted with sensors and/or microphones for feedback to therenderer, the microphone and sensor data 316 is input back to at leastone of the metadata processing component 306, the binaural renderingcomponent 310 or the audio post-processing component 314. For standardheadphones that are not fitted with sensors, headtracking could bereplaced by a simpler pseudo-randomly generated head ‘jitter’ thatmimics continuously changing small head movements. This allows anyrelevant environmental or operational data at the point of playback tobe used by the rendering system to further modify the audio tocounteract or enhance certain playback conditions.

As mentioned above, segmentation of the audio may be performed by theauthoring tool or the renderer. For the embodiment in which the audio ispre-segmented, the renderer processes this audio directly. FIG. 3B is ablock diagram of a rendering component used in an object-based headphonerendering system, under this alternative embodiment. As shown forrenderer 112 b, coded audio bitstream 321 from the authoring tool isprovided in its constituent parts of metadata 322 input to metadataprocessing component 326, and audio 324 to binaural rendering component330. For the embodiment of FIG. 3B, the audio is pre-segmented by anaudio pre-segmentation process 202 or 224 in the appropriate authoringtool. The binaural rendering component 330 performs certain headphonespecific rendering functions, such as 3D positioning, distance control,head size processing, and so on. The binaural rendered audio is theninput to audio post-processor 334, which applies certain audiooperations, such as level management, equalization, noise compensationor cancellation, and so on. The post-processed audio is then output 332for playback through headphones 116 or 118. For an embodiment in whichthe headphones or playback device 104 are fitted with sensors and/ormicrophones for feedback to the renderer, the microphone and sensor data336 is input back to at least one of the metadata processing component326, the binaural rendering component 330 or the audio post-processingcomponent 334. The authoring and rendering systems of FIGS. 2A, 2B, 3Aand 3B allow content authors to create and encode specific binauralrendering metadata at content creation time using authoring tool 102.This allows the audio data to be used to instruct the renderer toprocess parts of the audio content with different algorithms or withdifferent settings. In an embodiment, authoring tool 102 represents aworkstation or computer application that allows a content creator(author) to select or create audio content for playback and definecertain characteristics for each of the channels and/or objects thatmake up the audio content. The authoring tool may include a mixer typeconsole interface or a graphical user interface (GUI) representation ofa mixing console. FIG. 5 illustrates an authoring tool GUI that may beused with embodiments of a headphone rendering system, under anembodiment. As can be seen in GUI display 500, a number of differentcharacteristics are allowed to be set by the author such as gain levels,low frequency characteristics, equalization, panning, object positionand density, delays, fades, and so on. For the embodiment shown, userinput is facilitated by the use of virtual sliders for the author tospecify setting values, though other virtualized or direct input meansare also possible, such as direct text entry, potentiometer settings,rotary dials, and so on. At least some of the parameter settings enteredby the user are encoded as metadata that is associated with the relevantchannels or audio objects for transport with the audio content. In anembodiment, the metadata may be packaged as part of an additionalspecific headphone payload in the codec (coder/decoder) circuits in theaudio system. Using enabled devices, real-time metadata that encodescertain operational and environmental conditions (e.g., head tracking,head-size sensing, room sensing, ambient conditions, noise levels, etc.)can be provided live to the binaural renderer. The binaural renderercombines the authored metadata content and the real-time locallygenerated metadata to provide an optimized listening experience for theuser. In general, the object controls provided by the authoring toolsand user input interfaces allow the user to control certain importantheadphone-specific parameters, such as binaural and stereo-bypassdynamic rendering modes, LFE (low-frequency element) gain and objectgains, media intelligence and content-dependent controls. Morespecifically, rendering mode could be selected on a content-type basisor object basis between stereo (Lo/Ro), matrixed stereo (Lt/Rt), using acombination of interaural time delays and stereo amplitude or intensitypanning, or full binaural rendering (i.e. combination of interaural timedelays and levels as well as frequency dependent spectral cues). Inaddition, a frequency cross over point can be specified to revert tostereo processing below a given frequency. Low frequency gains can alsobe specified to attenuate low frequency components or LFE content. Lowfrequency content could also be transported separately to enabledheadphones, as described in greater detail below. Other metadata can bespecified on a per-content type or per-channel/object basis such as roommodel generally described by a direct/reverberant gain and a frequencydependent reverberation time and interaural target cross-correlation. Itcould also include other more detailed modeling of the room (e.g., earlyreflections positions, gains and late reverberation gain). It could alsoinclude directly specified filters modeling a particular room response.Other metadata includes warp to screen flags (that controls how objectsare remapped to fit screen aspect ratio and viewing angle as function ofdistance). Finally a listener relative flag (i.e., to apply headtrackinginformation or not), preferred scaling (specify a default size/aspectratio of ‘virtual room’ for rendering the content used to scale theobject positions as well as remap to screen (as a function of devicescreen size and distance to device)) as well as distance model exponentthat controls distance attenuation law (e.g., 1/(1+r^(α))) are alsopossible It is also possible to signal parameter groups or ‘presets’that can be applied to different channels/objects or depending oncontent-type.

As shown with respect to the pre-segmentation components of theauthoring tool and/or renderer, different types of content (e.g.,dialog, music, effects, etc.) may be processed differently based on theintent of the author and the optimum rendering configuration. Separationof content based on type or other salient characteristic can be achieveda priori during authoring, e.g. by manually keeping dialog separated intheir own set of tracks or objects, or a posteriori live prior torendering in the receiving device. Additional media intelligence toolscan be used during authoring to classify content according to differentcharacteristics and generate additional channels or objects that maycarry different sets of rendering metadata. For example, havingknowledge of the stems (music, dialog, Foley, effects, etc.) and anassociated surround (e.g., 5.1) mix, media classifiers could be trainedfor the content creation process to develop a model to identifydifferent stem mix proportions. An associated source separationtechnique could be employed to extract the approximate stems usingweighting functions derived from the media classifier. From theextracted stems, binaural parameters that would be encoded as metadatamay be applied during authoring. In an embodiment, a mirrored process isapplied in the end-user device whereby using the decoded metadataparameters would create a substantially similar experience as duringcontent creation.

In an embodiment, extensions to existing studio authoring tools includebinaural monitoring and metadata recording. Typical metadata captured atauthoring time include: channel/object position/size information foreach channel and audio object, channel/object gain adjustment, contentdependent metadata (can vary based on content type), bypass flags toindicate settings, such as stereo/left/right rendering should be usedinstead of binaural, crossover points and levels indicating that bassfrequency below cross over point must be bypassed and/or attenuated, androom model information to describe a direct/reverberant gain and afrequency dependent reverberation time or other characteristics, such asearly reflections and late reverberation gain. Other content dependentmetadata could provide warp to screen functionality that remaps imagesto fit screen aspect ratio or change the viewing angle as a function ofdistance. Head tracking information can be applied to provide a listenerrelative experience. Metadata could also be used that implements adistance model exponent that controls distance as a function ofattenuation law (e.g., 1/(1+r^(α)). These represent only certaincharacteristics that may be encoded by the metadata, and othercharacteristics may also be encoded.

FIG. 4 is a block diagram that provides an overview of the dual-endedbinaural rendering system, under an embodiment. In an embodiment, system400 provides content-dependent metadata and rendering settings thataffect how different types of audio content are to be rendered. Forexample, the original audio content may comprise different audioelements, such as dialog, music, effects, ambient sounds, transients,and so on. Each of these elements may be optimally rendered in differentways, instead of limiting them to be rendered all in only one way. Forthe embodiment of system 400, audio input 401 comprises a multi-channelsignal, object-based channel or hybrid audio of channel plus objects.The audio is input to an encoder 402 that adds or modifies metadataassociated with the audio objects and channels. As shown in system 400,the audio is input to a headphone monitoring component 410 that appliesuser adjustable parametric tools to control headphone processing,equalization, downmix, and other characteristics appropriate forheadphone playback. The user-optimized parameter set (M) is thenembedded as metadata or additional metadata by the encoder 402 to form abitstream that is transmitted to decoder 404. The decoder 404 decodesthe metadata and the parameter set M of the object and channel-basedaudio for controlling the headphone processing and downmix component406, which produces headphone optimized and downmixed (e.g., 5.1 tostereo) audio output 408 to the headphones. Although certain contentdependent processing has been implemented in present systems andpost-processing chains, it has generally not been applied to binauralrendering, such as illustrated in system 400 of FIG. 4 .

As shown in FIG. 4 , certain metadata may be provided by a headphonemonitoring component 410 that provides specific user adjustableparametric tools to control headphone-specific playback. Such acomponent may be configured to provide a user some degree of controlover headphone rendering for legacy headphones 118 that passivelyplayback transmitted audio content. Alternatively, the endpoint devicemay be an enabled headphone 116 that includes sensors and/or some degreeof processing capability to generate metadata or signal data that can beencoded as compatible metadata to further modify the authored metadatato optimize the audio content for rendering over headphones. Thus, atthe receiving end of the content, rendering is performed live and canaccount for locally generated sensor array data which can be generatedeither by a headset or an actual mobile device 104 to which headsets areattached, and such hardware-generated metadata can be further combinedwith the metadata created by the content creator at authoring time toenhance the binaural rendering experience.

As stated above, in some embodiments, low frequency content may betransported separately to enabled headphones allowing more than stereoinput (typically 3 or 4 audio inputs), or encoded and modulated into thehigher frequencies of the main stereo waveforms carried to a headsetwith only stereo input. This would allow further low frequencyprocessing to occur in the headphones (e.g. routing to specific driversoptimized for low frequencies). Such headphones may include lowfrequency specific drivers and/or filter plus crossover andamplification circuitry to optimize playback of low frequency signals.

In an embodiment, a link from the headphones to the headphone processingcomponent is provided on the playback side to enable manualidentification of the headphones for automatic headphone preset loadingor other configuration of the headphones. Such a link may be implementedas a wireless or wired link from the headphones to headphone process 406in FIG. 4 , for example. The identification may be used to configure thetarget headphones or to send specific content or specifically renderedcontent to a specific set of headphones if multiple target headphonesare being used. The headphone identifier may be embodied in anyappropriate alphanumeric or binary code that is processed by therendering process as either part of the metadata or a separate dataprocessing operation.

FIG. 6 illustrates an enabled headphone that comprises one or moresensors that sense playback conditions for encoding as metadata used ina headphone rendering system, under an embodiment. The various sensorsmay be arranged in a sensor array that can be used to provide livemetadata to the renderer at render time. For the example headphone 600of FIG. 6 , the sensors include a range sensor (such as infrared IR ortime-of-flight TOF camera) 602, tension/headsize sensor 604, gyroscopicsensor 606, external microphone (or pair) 610, ambient noisecancellation processor 608, internal microphone (or pair) 612, amongother appropriate sensors. As shown in FIG. 6 , the sensor array cancomprise both audio sensors (i.e., microphones) as well as data sensors(e.g., orientation, size, tension/stress, and range sensors).Specifically for use with headphones, orientation data can be used to‘lock’ or rotate the spatial audio object according to the listener'shead motion, tension sensors or external microphones can be used toinfer the size of the listener's head (e.g., by monitoring audio crosscorrelation at two external microphones located on the earcups) andadjust relevant binaural rendering parameters (e.g., interaural timedelays, shoulder reflection timing, etc.). Range sensors 602 can be usedto evaluate distance to the display in case of a mobile A/V playback andcorrect the location of on-screen objects to account for thedistance-dependent viewing angle (i.e. render objects wider as thescreen is brought closer to the listener) or adjust global gain and roommodel to convey appropriate distance rendering. Such a sensor functionis useful if the audio content is part of A/V content that is playedback on devices that may range from small mobile phones (e.g., 2-4″screen size) to tablets (e.g., 7-10″ screen size) to laptop computers(e.g., 15-17″ screen size). In addition sensors can also be used toautomatically detect and set the routing of the left and right audiooutputs to the correct transducers, not requiring a specific a prioriorientation or explicit “Left/Right” markings on the headphones.

As shown in FIG. 1 , the audio or A/V content transmitted to theheadphones 116 or 118 may be provided through a handheld or portabledevice 104. In an embodiment, the device 104 itself may include one ormore sensors. For example, if the device is a handheld game console orgame controller, certain gyro sensors and accelerometers may be providedto track object movement and position. For this embodiment, the device104 to which the headset is connected can also provide additional sensordata such as orientation, head size, camera, etc., as device metadata.

For this embodiment, certain headphone-to-device communication means areimplemented. For example, the headset can be connected to the deviceeither through a wired or wireless digital link or an analog audio link(microphone input), in which case the metadata will be frequencymodulated and added to the analog microphone input. FIG. 7 illustratesthe connection between a headphone and device 104 including a headphonesensor processor 702, under an embodiment. As shown in system 700,headphone 600 transmits certain sensor, audio and microphone data 701over a wired or wireless link to headphone sensor processor 702. Theprocessed data from processor 702 may comprise analog audio withmetadata 704 or spatial audio output 706. As shown in FIG. 7 , each ofthe connections comprises a bi-directional link between the headphone,processor, and outputs. This allows sensor and microphone data to betransmitted to and from the headphones and device for creation ormodification of appropriate metadata. In addition to hardware generatedmetadata, user controls can also be provided to complement or generateappropriate metadata if not available through hardware sensor arrays.Example user controls can include: elevation emphasis, binaural on/offswitch, preferred sound radius or size, and other similarcharacteristics. Such user controls may be provided through hardware orsoftware interface elements associated with the headphone processorcomponent, playback device, and/or headphones.

FIG. 8 is a block diagram illustrating the different metadata componentsthat may be used in a headphone rendering system, under an embodiment.As shown in diagram 800, the metadata processed by the headphoneprocessor 806 comprises authored metadata, such as that produced byauthoring tool 102 and mixing console 500, and hardware generatedmetadata 804. The hardware generated metadata 804 may include user inputmetadata, device-side metadata provided by or generated from data sentfrom device 808, and/or headphone-side metadata provided by or generatedfrom data sent from headphone 810.

In an embodiment, the authored 802 and/or hardware-generated 804metadata is processed in a binaural rendering component 114 of renderer112. The metadata provides control over specific audio channels and/orobjects to optimize playback over headphones 116 or 118. FIG. 9illustrates functional components of a binaural rendering component forheadphone processing, under an embodiment. As shown in system 900,decoder 902 outputs the multi-channel signal or the channel plus objecttracks along with the decoded parameter set, M, for controlling theheadphone processing performed by headphone processor 904. The headphoneprocessor 904 also receives certain spatial parameter updates 906 fromcamera-based or sensor-based tracking device 910. Tracking device 910 isa face-tracking or head-tracking device that measures certain angularand positional parameters (r, θ, ϕ) associated with the user's head. Thespatial parameters may correspond to distance and certain orientationangles, such as yaw, pitch, and roll. An original set of spatialparameters, x, may be updated as the sensor data 910 is processed. Thesespatial parameters updates Y are then passed to the headphone processor904 for further modification of the parameter set M. The processed audiodata is then transmitted to a post-processing stage 908 that performscertain audio processing such as timbre-correction, filtering,downmixing, and other relevant processes. The audio is then equalized byequalizer 912 and transmitted to the headphones. In an embodiment, theequalizer 912 may perform equalization with or without using apressure-division-ratio (PDR) transform, as described in further detailin the description that follows.

FIG. 10 illustrates a binaural rendering system for rendering audioobjects in a headphone rendering system, under an embodiment. FIG. 10illustrates some of the signal components as they are processed througha binaural headphone processor. As shown in diagram 1000, object audiocomponents are input to an unmixer 1002 that separates direct anddiffuse components (e.g., direct from reverb path) of the audio. Thedirect component is input to a downmix component 1006 that downmixessurround channels (e.g., 5.1 surround) to stereo with phase shiftinformation. The direct component is also input to a direct contentbinaural renderer 1008. Both two-channel components are then input to adynamic timbre equalizer 1012. For the object based audio input, theobject position and user control signals are input to a virtualizersteerer component 1004. This generates a scaled object position that isinput to the binaural renderer 1008 along with the direct component. Thediffuse component of the audio is input to a separate binaural renderer1010, and is combined with the rendered direct content by an addercircuit prior to output as two-channel output audio.

FIG. 11 illustrates a more detailed representation of the binauralrendering system of FIG. 10 , under an embodiment. As shown in diagram1100 of FIG. 11 , the multi-channel and object-based audio is input tounmixer 1102 for separation into direct and diffuse components. Thedirect content is processed by direct binaural renderer 1118, and thediffuse content is processed by diffuse binaural renderer 1120. Afterdownmixing 1116 and timbre equalization 1124 of the direct content thediffuse and direct audio components are then combined through an addercircuit for post-processing, such as by headphone equalizer 1122, andother possible circuits. As shown in FIG. 11 , certain user input andfeedback data are used to modify the binaural rendering of the diffusecontent in diffuse binaural renderer 1120. For the embodiment of system1100, playback environment sensor 1106 provides data regarding listeningroom properties and noise estimation (ambient sound levels), head/facetracking sensor 1108 provides head position, orientation, and size data,device tracking sensor 1110 provides device position data, and userinput 1112 provides playback radius data. This data may be provided bysensors located in the headphone 116 and/or device 104. The varioussensor data and user input data is combined with content metadata, whichprovides object position and room parameter information in a virtualizersteerer component 1104. This component also receives direct and diffuseenergy information from the unmixer 1102. The virtualizer steerer 1104outputs data including object position, head position/orientation/size,room parameters, and other relevant information to the diffuse contentbinaural renderer 1120. In this manner, the diffuse content of the inputaudio is adjusted to accommodate sensor and user input data.

While optimal performance of the virtualizer steerer is achieved whensensor data, user input data, and content metadata are received, it ispossible to achieve beneficial performance of the virtualizer steerereven in the absence of one or more of these inputs. For example, whenprocessing legacy content (e.g., encoded bitstreams which do not containbinaural rendering metadata) for playback over conventional headphones(e.g., headphones which do not include various sensors, microphones,etc.), a beneficial result may still be obtained by providing the directenergy and diffuse energy outputs of the unmixer 1102 to the virtualizersteerer 1104 to generate control information for the diffuse contentbinaural renderer 1120, even in the absence of one or more other inputsto the virtualizer steerer.

In an embodiment, rendering system 1100 of FIG. 11 allows the binauralheadphone renderer to efficiently provide individualization based oninteraural time difference (ITD) and interaural level difference (ILD)and sensing of head size. ILD and ITD are important cues for azimuth,which is the angle of an audio signal relative to the head when producedin the horizontal plane. ITD is defined as the difference in arrivaltime of a sound between two ears, and the ILD effect uses differences insound level entering the ears to provide localization cues. It isgenerally accepted that ITDs are used to localize low frequency soundand ILDs are used to localize high frequency sounds, while both are usedfor content that contains both high and low frequencies.

Rendering system 1100 also allows accommodation for source distancecontrol and room model. It further allows for direct versusdiffuse/reverb (dry/wet) content extraction and processing, optimizationof room reflections, and timbral matching.

HRTF Model

In spatial audio reproduction, certain sound source cues arevirtualized. For example, sounds intended to be heard from behind thelisteners may be generated by speakers physically located behind them,and as such, all of the listeners perceive these sounds as coming frombehind. With virtual spatial rendering over headphones, on the otherhand, perception of audio from behind is controlled by head relatedtransfer functions (HRTF) that are used to generate the binaural signal.In an embodiment, the metadata-based headphone processing system 100 mayinclude certain HRTF modeling mechanisms. The foundation of such asystem generally builds upon the structural model of the head and torso.This approach allows algorithms to be built upon the core model in amodular approach. In this algorithm, the modular algorithms are referredto as ‘tools.’ In addition to providing ITD and ILD cues, the modelapproach provides a point of reference with respect to the position ofthe ears on the head, and more broadly to the tools that are built uponthe model. The system could be tuned or modified according toanthropometric features of the user. Other benefits of the modularapproach allow for accentuating certain features in order to amplifyspecific spatial cues. For instance, certain cues could be exaggeratedbeyond what an acoustic binaural filter would impart to an individual.FIG. 12 is a system diagram showing the different tools used in an HRTFmodeling system used in a headphone rendering system, under anembodiment. As shown in FIG. 12 , certain inputs including azimuth,elevation, fs, and range are input to modeling stage 1204, after atleast some input components are filtered 1202. In an embodiment, filterstage 1202 may comprise a snowman filter model that consists of aspherical head on top of a spherical body and accounts for thecontributions of the torso as well as the head to the HRTF. Modelingstage 1204 computes the pinna and torso models and the left and right(1, r) components are post-processed 1206 for final output 1208.

Metadata Structure

As described above, the audio content processed by the headphoneplayback system comprises channels, objects and associated metadata thatprovides the spatial and processing cues necessary to optimize renderingof the audio through headphones. Such metadata can be generated asauthored metadata from authoring tools as well as hardware generatedmetadata from one or more endpoint devices. FIG. 13 illustrates a datastructure that enables delivery of metadata for a headphone renderingsystem, under an embodiment. In an embodiment, the metadata structure ofFIG. 13 is configured to supplement the metadata delivered in otherportions of a bitstream that may be packaged in accordance with a knownchannel-based audio format, such as Dolby digital AC-3 or Enhanced AC-3bit stream syntax. As shown in FIG. 13 , the data structure consists ofa container 1300 that contains one or more data payloads 1304. Eachpayload is identified in the container using a unique payload identifiervalue to provide an unambiguous indication of the type of data presentin the payload. The order of payloads within the container is undefined.Payloads can be stored in any order, and a parser must be able to parsethe entire container to extract relevant payloads, and ignore payloadsthat are either not relevant or are unsupported. Protection data 1306follows the final payload in the container that can be used by adecoding device to verify that the container and the payloads within thecontainer are error-free. A preliminary portion 1302 containing sync,version, and key-ID information precedes the first payload in thecontainer.

The data structure supports extensibility through the use of versioningand identifiers for specific payload types. The metadata payloads may beused to describe the nature or configuration of the audio program beingdelivered in an AC-3 or Enhanced AC-3 (or other type) bit stream, or maybe used to control audio processing algorithms that are designed tofurther process the output of the decoding process.

Containers may be defined using different programming structures, basedon implementation preferences. The table below illustrates examplesyntax of a container, under an embodiment.

container( ) { Version..................................................................2  if (version == 3)  {   version +=variable_bits(2)............................. variable_bits(2)  } key_id...................................................................3  if (key_id == 7)  {   key_id +=variable_bits(3).............................. variable_bits(3)  } payload_id...............................................................5  while (payload_id != 0x0)  {   if (payload_id == 0x1F)   {   payload_id += variable_bits(5).......................variable_bits(5)   }   payload_config( )  payload_size............................................variable_bits(8)   for (i = 0; i < payload_size; i++)   {   payload_bytes.......................................................8   }  }  protection( ) }

An example of possible syntax of the variable bits for the examplecontainer syntax provided above is shown in the following table:

Syntax variable_bits (n_bits) {  value = 0;  Do  {   value +=read......................................................................n_bits  read_more...............................................................................1   if (read_more)   {    value <<= n_bits;    value += (1<<n_bits);   } }  while (read_more);  return value; }

An example of possible syntax of the payload configuration for theexample container syntax provided above is shown in the following table:

Syntax No. of bits payload_config( ) { Smploffste.................................................................................1  if(smploffste){smploffst}................................................................11 Reserved...................................................................................1 Duratione..................................................................................1  if(duratione){duration}...................................................variable_bits(11) Groupide...................................................................................1  if (groupide){groupid}.....................................................variable_bits(2) Codecdatae.................................................................................1  if (codecdatae){reserved}.................................................................8 discard_unknown_payload....................................................................1  if(discard_unknown_payload == 0)  {   if (smploffste == 0)   {   payload_frame_aligned................................................................1    if (payload_frame_aligned)    {    create_duplicate..................................................................1    remove_duplicate..................................................................1    }   }   if (smpoffste == 1 || payload_frame_aligned == 1)   {   Priority.............................................................................5   proc_allowed.........................................................................2   }  } }

The above syntax definitions are provided as example implementations,and are not meant to be limiting as many other different programstructures may be used. In an embodiment, a number of fields within thecontainer structure and payload data are encoded using a method known asvariable-bits. This method enables efficient coding of small fieldvalues with extensibility to be able to express arbitrarily large fieldvalues. When variable_bit coding is used, the field is consists of oneor more groups of n-bits, with each group followed by a 1-bit read_morefield. At a minimum, coding of n bits requires n+1 bits to betransmitted. All fields coded using variable_bits are interpreted asunsigned integers. Various other different coding aspects may beimplemented according to practices and methods known to those ofordinary skill in the art. The above tables and FIG. 13 illustrate anexample metadata structure, format, and program content. It should benoted that these are intended to represent one example embodiment of ametadata representation, and other metadata definitions and content arealso possible.

Headphone EQ and Correction

As illustrated in FIG. 1 , certain post-processing functions 115 may beperformed by the renderer 112. One such post-processing functioncomprises headphone equalization, as shown in element 912 of FIG. 9 . Inan embodiment, equalization may be performed by obtaining blocked-earcanal impulse response measurements for different headphone placementsfor each ear. FIG. 14 illustrates an example case of three impulseresponse measurements for each ear, in an embodiment of a headphoneequalization process. The equalization post-process compute the FastFourier Transform (FFT) of each response and performs an RMS (root-meansquared) averaging of the derived response. The responses may bevariable, octave smoothed, ERB smoothed, etc. The process then computesthe inversion, |F(ω)|, of the RMS average with constraints on the limits(+/−x dB) of the inversion magnitude response at mid- andhigh-frequencies. The process then determines the time-domain filter.

The post-process may also include a closed-to-open transform function.This pressure-division-ratio (PDR) method involves designing a transformto match the acoustical impedance between eardrum and free-field forclosed-back headphones with modifications in terms of how themeasurements are obtained for free-field sound transmission as afunction of direction of arrival first-arriving sound. This indirectlyenables matching the eardrum pressure signals between closed-backheadphones and free-field equivalent conditions without requiringcomplicated eardrum measurements.

FIG. 15A illustrates a circuit for calculating the free-field soundtransmission, under an embodiment. Circuit 1500 is based on a free-fieldacoustical impedance model. In this model, P₁(ω) is the Theveninpressure measured at the entrance of the blocked ear canal with aloudspeaker at 0 degrees about the median plane (e.g., about 30 degreesto the left and front of the listener) involving extraction of directsound from the measured impulse response. Measurement P₁(ω) can be doneat the entrance of the ear canal or at a certain distance inside (x mm)inside the ear canal (or at the eardrum) from the opening for the sameloudspeaker at the same placement for measuring P₁(ω) involvingextraction of direct sound from the measured impulse response.

For this model, the ratio of P₂(ω)/P₁(ω) is calculated as follows:

$\frac{P_{2}(\omega)}{P_{1}(\omega)} = \frac{Z_{eardrum}(\omega)}{{Z_{eardrum}(\omega)} + {Z_{radiation}(\omega)}}$

FIG. 15B illustrates a circuit for calculating the headphone soundtransmission, under an embodiment. Circuit 1510 is based on a headphoneacoustical impedance analog model. In this model, P₄ is measured at theentrance of the blocked ear canal with headphone (RMS averaged)steady-state measurement, and measure P₅(ω) is made at the entrance tothe ear canal or at a distance inside the ear canal (or at the eardrum)from the opening for the same headphone placement used for measuringP₄(ω)).

For this model, the ratio of P₅(ω)/P₄(ω) is calculated as follows:

$\frac{P_{5}(\omega)}{P_{4}(\omega)} = \frac{Z_{eardrum}(\omega)}{{Z_{eardrum}(\omega)} + {Z_{headphone}(\omega)}}$

The pressure-division-ratio (PDR) can then be calculated using thefollowing formula:

${{PDR}(\omega)} = {\frac{P_{2}(\omega)}{P_{1}(\omega)} \div \frac{P_{5}(\omega)}{P_{4}(\omega)}}$

Aspects of the methods and systems described herein may be implementedin an appropriate computer-based sound processing network environmentfor processing digital or digitized audio files. Portions of theadaptive audio system may include one or more networks that comprise anydesired number of individual machines, including one or more routers(not shown) that serve to buffer and route the data transmitted amongthe computers. Such a network may be built on various different networkprotocols, and may be the Internet, a Wide Area Network (WAN), a LocalArea Network (LAN), or any combination thereof. In an embodiment inwhich the network comprises the Internet, one or more machines may beconfigured to access the Internet through web browser programs.

One or more of the components, blocks, processes or other functionalcomponents may be implemented through a computer program that controlsexecution of a processor-based computing device of the system. It shouldalso be noted that the various functions disclosed herein may bedescribed using any number of combinations of hardware, firmware, and/oras data and/or instructions embodied in various machine-readable orcomputer-readable media, in terms of their behavioral, registertransfer, logic component, and/or other characteristics.Computer-readable media in which such formatted data and/or instructionsmay be embodied include, but are not limited to, physical(non-transitory), non-volatile storage media in various forms, such asoptical, magnetic or semiconductor storage media.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in a sense of “including,but not limited to.” Words using the singular or plural number alsoinclude the plural or singular number respectively. Additionally, thewords “herein,” “hereunder,” “above,” “below,” and words of similarimport refer to this application as a whole and not to any particularportions of this application. When the word “or” is used in reference toa list of two or more items, that word covers all of the followinginterpretations of the word: any of the items in the list, all of theitems in the list and any combination of the items in the list.

While one or more implementations have been described by way of exampleand in terms of the specific embodiments, it is to be understood thatone or more implementations are not limited to the disclosedembodiments. To the contrary, it is intended to cover variousmodifications and similar arrangements as would be apparent to thoseskilled in the art. Therefore, the scope of the appended claims shouldbe accorded the broadest interpretation so as to encompass all suchmodifications and similar arrangements.

The invention claimed is:
 1. A method, performed by an audio signalprocessing device, for generating a binaural rendering of digital audiocontent for playback through headphones, the method comprising:receiving an encoded signal comprising the digital audio content andrendering metadata, wherein the digital audio content comprises aplurality of audio object signals and channel audio signals; receivingplayback control metadata comprising room model metadata; decoding theencoded signal to obtain the plurality of audio object signals andchannel audio signals; and generating the binaural rendering of thedigital audio content in response to the plurality of audio objectsignals and channel audio signals, the rendering metadata, and theplayback control metadata; wherein the rendering metadata indicates, foreach audio object signal, position and gain, and, for each channel audiosignal, a position of the channel audio signal, and whether to usestereo instead of binaural rendering for the channel audio signal;wherein, when the rendering metadata indicates, for a channel audiosignal, to use stereo rendering for the channel audio signal, generatingthe binaural rendering of the digital audio content comprises, for thechannel audio signal, generating a stereo rendering of the channel audiosignal; and wherein, when the rendering metadata indicates, for achannel audio signal, to use binaural rendering for the channel audiosignal, generating the binaural rendering of the digital audio contentcomprises, for the channel audio signal, generating a binaural renderingof the channel audio signal.
 2. An audio signal processing device forgenerating a binaural rendering of digital audio content for playbackthrough headphones, the audio signal processing device comprising one ormore processors to: receive an encoded signal comprising the digitalaudio content and rendering metadata, wherein the digital audio contentcomprises a plurality of audio object signals and channel audio signals;receive playback control metadata comprising room model metadata; decodethe encoded signal to obtain the plurality of audio object signals andchannel audio signals; and generate the binaural rendering of thedigital audio content in response to the plurality of audio objectsignals and channel audio signals, the rendering metadata, and theplayback control metadata; wherein the rendering metadata indicates, foreach audio object signal, position and gain, and, for each channel audiosignal, a position of the channel audio signal, and whether to usestereo instead of binaural rendering for the channel audio signal;wherein, when the rendering metadata indicates, for a channel audiosignal, to use stereo rendering for the channel audio signal, generatingthe binaural rendering of the digital audio content comprises, for thechannel audio signal, generating a stereo rendering of the channel audiosignal; and wherein, when the rendering metadata indicates, for achannel audio signal, to use binaural rendering for the channel audiosignal, generating the binaural rendering of the digital audio contentcomprises, for the channel audio signal, generating a binaural renderingof the channel audio signal.
 3. A non-transitory computer readablestorage medium comprising a sequence of instructions, which, whenexecuted by an audio signal processing device, cause the audio signalprocessing device to perform a method for generating a binauralrendering of digital audio content for playback through headphones, themethod comprising: receiving an encoded signal comprising the digitalaudio content and rendering metadata, wherein the digital audio contentcomprises a plurality of audio object signals and channel audio signals;receiving playback control metadata comprising room model metadata;decoding the encoded signal to obtain the plurality of audio objectsignals and channel audio signals; and generating the binaural renderingof the digital audio content in response to the plurality of audioobject signals and channel audio signals, the rendering metadata, andthe playback control metadata; wherein the rendering metadata indicates,for each audio object signal, position and gain, and, for each channelaudio signal, a position of the channel audio signal, and whether to usestereo instead of binaural rendering for the channel audio signal;wherein, when the rendering metadata indicates, for a channel audiosignal, to use stereo rendering for the channel audio signal, generatingthe binaural rendering of the digital audio content comprises, for thechannel audio signal, generating a stereo rendering of the channel audiosignal; and wherein, when the rendering metadata indicates, for achannel audio signal, to use binaural rendering for the channel audiosignal, generating the binaural rendering of the digital audio contentcomprises, for the channel audio signal, generating a binaural renderingof the channel audio signal.